6 is the usual number, however if you do want to capitalize on larger cities than 6 specialty cities (when a single city isn't producing enough prosperity, etc etc) you may wish to kill off your 6th so your five larger cities will expand that way.

I've been having fun experimenting with getting 7 stable cities in a 2 hour medium game. Managed to get an Opera House, 2 Observatories, an Apothecary, a Bank, a Hamlet and a Workshop. Was very close to creating a Canal Town which would have finished off my Giants, but I got distracted by an impossible Hospital so didn't spot the very possible Canal Town.
On free play, the Canal Town was finished. I should definitely be able to create a Multinational, and a Factory / Opera House / second Multinational. The Hospital is going to require a bit of luck I fear, as it is in a very cramped location. I should be able to create one of the Laboratories, but the other will require some serious tectonic engineering as currently there is a harbour and three squares of dry land.

Went for an animal based build. Ocean had Swamp, Swamp, Forest, (Forest). Forest had Forest, Swamp, Desert and another Desert. Rock had Desert, Swamp, Forest and Forest. Swamp had Forest, Swamp, Desert and Forest.

Managed to improve it, but it got very confusing for both me and my cities! In the two hours, I had: Laboratory, Island Town, Canal City. Observatory, and two Circuses. And a Workshop. Same build on the Giants as before. The confusing aspect was that I had a desert Laboratory following a terrain change, and a Canal city on what was formally a trade post owning desert city. I had made the same mistake as before by not leaving enough land for minerals and plants for the Laboratory, so I had to remove the ocean supplying the swamp. This created a desert city and wiped out the food supply. I created a mini mountain, placed a Fluorite on top of that mountain, created a token swamp tile and mined it for Uranium. Finally, three Superior Javelinas provided the food and a lot of danger to keep the Lab intact.

The build still works. By focusing on animals it gives ready access to Danger to keep cities under control.

Third time, slightly better. Island Town, Castle, Factory. Opera would have been completed in the next 3 minutes. Other than that had another circus and an apothecary. The other town was still level two as it kept insisting on building historic points. I think it was a Shrine.

The thrid project has no string attached, so it, again, can be any project from the biome. But the 4th is also related with the third, and usually, there is only one kind of 4th one (before the Patch, there was only one, but now with Town Canal this isn't true anymore).

For instance; Observatory will always upgrate to Laboratory; Blast Furnace will become a Factory; Circus will become an Opera House, Bank a Multinational, and so on.

So although there is a random component, you will have a very limited options of projects in the 2nd and 4th level an can plan ahead!